In the field of semi-conductor processing, various types of processing equipment are now in widespread use which provide for automated handling of wafers in the vacuum chambers where the processing is carried out. Common semi-conductor processes performed in vacuum environments include the deposition of metallic layers on the surface of the wafer, sometimes known as physical vapor deposition (PVD) or sputtering, selective epitaxial deposition, chemical vapor deposition (CVD), plasma etching, ion implantation, etc. These processes typically involve the use of one or more load-lock vacuum chambers in which the wafers are processed, following which they are removed from the chambers.
At a process station within the chamber, is often necessary to hold the wafer firmly against a support surface, typically a heating assembly, in order to maintain the position of the wafer relative to the processing equipment, or to maintain good thermal contact with the heating assembly. A common technique for maintaining the temperature of a wafer undergoing processing in a vacuum environment is to introduce a conductive gas in a narrow space at the back side of the wafer, thereby thermally coupling the wafer to a temperature control element. When using a backside gas, which is introduced at a pressure higher than the ambient pressure within the processing chamber, clamping means are required to ensure that the backside gas does not move the wafer off of the support surface.
In many cases, the particular process used in the process chamber may cause the clamp to adhere to a wafer after completion of a processing step, thus preventing the wafer from being picked up by a robot arm or other transport mechanism. This may occur, for example, when depositing a metal layer over a wafer which causes the wafer to stick to the clamp, or where a top layer of material on the wafer melts or becomes plastic at an elevated temperature used in a processing step, thus causing the top layer to adhere to the clamp. In some cases, repeatedly deposited film, such as a photoresist, accumulates on surfaces of the clamp, thus also tending to cause the wafer to adhere to the clamp.
When a wafer sticks to a clamp, the entire processing system typically must be shut down to free the stuck wafer. This remedial procedure requires the process chamber to be vented and partially disassembled so that the wafer can be manually dislodged from the clamp. This procedure can require several hours of time, and with the additional time required to evacuate and cleanse the chamber, a stuck wafer can result in down time of four to five hours. This naturally reduces wafer throughput of the entire processing system, and results in corresponding revenue losses.
Others in the past have attempted various solutions to the wafer sticking problem described above. For example, one approach involves providing a slightly beveled surface on the clamp so that only the outer most edge of the wafer is contacted, as far as from the deposition process as possible. An overhang or "hood" is formed by the projection of the beveled edge which tends to block sputtered material from reaching the area where the clamp makes physical contact with the wafer. This beveled surface or hood does not, however, completely eliminate the possibility of sticking, because after processing a series of batches, the deposited material tends to accumulate, eventually building up back into the area where the clamp contacts the wafer's edge. Even more serious are those instances where certain films have been deposited onto the entire surface of the wafer and the clamp is applied directly over the deposited film in a subsequent processing step, thus bringing the film and clamping surface into direct contact with each other.
Another attempt at solving the wafer sticking problem is described in U.S. Pat. No. 5,513,594 issued May 7, 1996 to McClanahan. The McClanahan patent discloses the use of a spring-loaded releasing mechanism which applies a biasing force tending to force the wafer away from the clamp upon the release of the clamp. This solution is not entirely satisfactory, however, because of the need to use small springs and mechanisms which add to the complexity for the clamp and potentially reduce reliability. Moreover, spring-loaded release of the wafer from the clamp can result in damage to the wafer or dislodgement of deposited particles that are dispersed into the chamber and can may interfere with subsequent processing steps.
It is therefore a primary object invention to provide a novel semi-conductor wafer clamp which resists adhering to a wafer, thus overcomes the shortcomings of the prior art.
Another object of the present invention is to provide a semi-conductor wafer clamp as described above which provides for ready release of wafers without the need for additional structure or complex mechanisms.
Another object of the present invention is to provide a semi-conductor wafer clamp which is effective in releasing wafers in spite of the fact that deposited films and materials come into mutual contact with both the wafer and the clamp.
A further object of the present invention is to provide a novel method for preventing a wafer clamp from adhering to deposited materials, while also preventing build-up on the clamp of a film formed from the deposited materials.
As a corollary of the foregoing object, another object of the invention is to provide a method of treating at least portions of the surfaces of a semi-conductor wafer clamp which substantially reduces the tendency of the clamp surfaces to stick to the wafer. These, and further objects and advantages of the invention will be made clear or will become apparent during the course of the following description of a preferred embodiment of the present invention.